1. Field of the Disclosure
This invention pertains to a printing form precursor and a method for preparing a printing form from the precursor, and particularly a printing form precursor that includes at least one indicia of text or graphic that is present in the printing form.
2. Description of Related Art
Flexographic printing plates are widely used for printing of packaging materials ranging from corrugated carton boxes to card boxes and to continuous web of plastic films. Flexographic printing plates can be prepared from photopolymerizable compositions, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,759. The photopolymerizable compositions generally comprise an elastomeric binder, at least one monomer and a photoinitiator. Photosensitive elements generally have a layer of the photopolymerizable composition interposed between a support and a coversheet or a multilayer cover element.
Photopolymerizable elements for flexographic printing are characterized by their ability to crosslink or cure upon exposure to actinic radiation. Photopolymerizable elements undergo a multi-step process and typically involve at least three separate exposures to actinic radiation, to be converted into useful printing forms. In most instances, the actinic radiation is ultraviolet radiation (UV) or visible light. The photosensitive element is uniformly exposed through the backside of the plate to actinic radiation to create a floor or layer of cured photopolymer adjacent the support. Next, an imagewise exposure by actinic radiation of the front-side of the element is made through an image-bearing artwork or a template, such as a photographic negative or transparency (e.g. silver halide film) or through an in-situ mask having radiation opaque areas that had been previously formed above the photopolymerizable layer. The actinic radiation enters the photopolymerizable layer through the clear areas of the transparency and is blocked from entering the black or opaque areas. The exposed material crosslinks and becomes insoluble to solvents used during relief image development. The unexposed photopolymerizable areas under the opaque regions of the transparency are not cross-linked and remain soluble, and are then washed away with a suitable solution, i.e., solvent or aqueous-based, leaving a relief image suitable for printing. Alternatively, a “dry” thermal development process may be used to form the relief image in which the imagewise exposed photosensitive layer is contacted with an absorbent material at a temperature sufficient to cause the composition in the unexposed portions of the photopolymerizable layer to soften or melt and flow into the absorbent material. See U.S. Pat. No. 3,264,103 (Cohen et al.); U.S. Pat. No. 5,015,556 (Martens); U.S. Pat. No. 5,175,072 (Martens); U.S. Pat. No. 5,215,859 (Martens); and U.S. Pat. No. 5,279,697 (Peterson et al.). The exposed portions of the photopolymerizable layer remain hard, that is, do not soften or melt, at the softening temperature for the unexposed portions. The absorbent material collects the softened un-irradiated material and then is separated and/or removed from the photopolymer layer. The cycle of heating and contacting the photopolymer layer may be repeated several times in order to sufficiently remove the un-irradiated areas and form a relief structure suitable for printing. The printing plate can be further treated to remove surface tackiness. After all desired processing steps, the plate is mounted on a cylinder and used for printing.
A plethora of photopolymerizable elements for flexographic printing exist due to the number of manufacturers as well as the variety of products provided by each of the manufacturers to address particular needs in end-use situations. Manufacturers have included a dye or other coloring agent in the photopolymerizable composition for customers to easily identify and to distinguish their products from other manufacturers' products. Customers have thus associated color of the photopolymerizable element precursor with certain expectations of quality and performance of the resulting printing form.
A problem exists in that counterfeit photopolymerizable print precursors are being made that have the same or similar color as the color of photopolymerizable print precursors having established quality and performance standards in the industry. In the printing industry, printers often use the services of trade shops to prepare printing forms from photopolymerizable elements. Printers often request a particular manufacturer's printing form. But with nothing other than the color as identification of the printing form, counterfeit printing forms can be, and are being, substituted for the printing forms having established quality and performance standards.
From an end user's standpoint, e.g., trade shops and converters, it is desirable to be able to identify the photopolymerizable element should it become separated from its packaging. From the manufacturer's standpoint, when assisting the end-users and/or accepting a returned product, it is desirable to verify that the product is in fact one made by the manufacturer, and not a counterfeit product from another manufacturer. Thus, it is desirable for users to be able to authenticate the identity of the photopolymerizable element, and more importantly for printers to authenticate the identity of the resulting printing form while on press.
Other than color there has been no useful method by which a photopolymerizable element, or its resulting printing form, can be identified, particularly by a printer who has not been directly involved in the printing form preparation. Any identification should survive the multi-step process to convert the photopolymerizable element to a printing form, as well as be sufficiently viewable on the printing form when mounted on a print cylinder of a print press. Also the presence of an identifier on the photopolymerizable element should not influence the properties of the photopolymerizable element, or the creation of the print surface, e.g., relief surface, on the resulting printing form, by the multi-step conversion process to the extent that print characteristics and/or functionality of the printing form are detrimentally impacted. Furthermore in some end-use applications, some printing forms sold in a large size by the manufacturer are cut into smaller sizes by customers, e.g., printers, converters, trade shops, for customized print jobs and for more efficient use of materials. The smaller sized printing forms in some cases are mounted onto carrier sheets that are placed on print cylinders. As such, the size and spacing of the identification should be designed in such a way that the identification is retained with the customized smaller-sized printing forms, and is sufficiently viewable on the when the printing form is mounted onto the carrier sheet.
Currently, one manufacturer marks the coversheet of a photopolymerizable printing precursor with a stripe of repeating textual identification information, by what appears to be dot matrix application. However, the coversheet is removed early in the multi-step conversion process, and as such, the identity of the precursor is not retained through the multi-step conversion process, and not within the resulting printing form. Another manufacturer marks the support of the photopolymerizable printing form, by what appears to be laser etching, with textual product identification information that is approximate and parallel to a side edge of the precursor. The information is so small and close to the side edge that it would typically be captured in the strip that is cut to remove non-imaged edges of the printing form; and, thus would not be present to identify the printing form when mounted on the print cylinder. Even if the information survived edge trimming, this mark would difficult to see once the printing form is mounted onto a print cylinder. Also, in the event that the photopolymerizable element is cut into smaller size printing forms by the customer, the information would not be included on all the smaller size printing forms.
It is not uncommon for manufacturers to mark their goods with identifying marks or brand names as a way to identify and for consumers to distinguish their products from similar goods from other manufacturers. However, the marking of photosensitive printing elements to identify and distinguish the elements sufficiently so that the marking is retained throughout its end-use has been generally avoided because of the difficulties that marking imposes on the conversion of the photosensitive element to the printing form.
Thus, it is desirable to be able to uniquely identify the photopolymerizable element by more than its color so that end-users, e.g., trade shops and printers, can view the identification and be assured of the origin of the resulting printing form. The identification should survive the multi-step process for conversion from the photosensitive element to a printing form, and retain the identification in the resulting printing form.